PHP 多进程的困惑

最近在研究 php 多进程。看到普遍的观点是，多进程对于单核 cpu 没有什么作用。因为 cpu 同时只能运行一个线程，当然同时只能运行一个进程，那么进程调度的开销让你的多进程没有意义。 可是实际运行却发现，多进程比单进程耗时要小的多。请看代码：

<?php
class WebServer
{
private $list;
public function __construct()
{
$this->list = [];
}
public function worker($request){
$pid = pcntl_fork();
if($pid == -1){
return false;
}
if($pid > 0){
return $pid;
}
if($pid == 0){
$time = $request[0];
$method = $request[1];
$start = microtime(true);
echo getmypid()."\t start ".$method."\tat".$start.PHP_EOL;
//sleep($time);
$c = file_get_contents($method);
echo getmypid() ."\n";
$end = microtime(true);
$cost = $end-$start;
echo getmypid()."\t stop \t".$method."\tat:".$end."\tcost:".$cost.PHP_EOL;
exit(0);
}
}
public function master($requests){
$start = microtime(true);
echo "All request handle start at ".$start.PHP_EOL;
foreach ($requests as $request){
$pid = $this->worker($request);
if(!$pid){
echo 'handle fail!'.PHP_EOL;
return;
}
array_push($this->list,$pid);
}
while(count($this->list)>0){
foreach ($this->list as $k=>$pid){
$res = pcntl_waitpid($pid,$status,WNOHANG);
if($res == -1 || $res > 0){
unset($this->list[$k]);
}
}
usleep(100);
}
$end = microtime(true);
$cost = $end - $start;
echo "All request handle stop at ".$end."\t cost:".$cost.PHP_EOL;
}
}

$requests = [
[1,'http://www.sina.com'],
[2,'http://www.sina.com'],
[3,'http://www.sina.com'],
[4,'http://www.sina.com'],
[5,'http://www.sina.com'],
[6,'http://www.sina.com']
];

echo "多进程测试：".PHP_EOL;
$server = new WebServer();
$server->master($requests);

echo PHP_EOL."单进程测试：".PHP_EOL;
$start = microtime(true);
for($i=0;$i<6;$i++){
$c = file_get_contents("http://www.sina.com");
}
$end = microtime(true);
$cost = $end - $start;
echo "All request handle stop at ".$end."\t cost:".$cost.PHP_EOL;

-------------------------------------------------------------------------------------
测试结果如下：
多进程测试：
All request handle start at 1524498288.8329
17820 start http://www.sina.com at1524498288.8379
17821 start http://www.sina.com at1524498288.8383
17822 start http://www.sina.com at1524498288.8389
17823 start http://www.sina.com at1524498288.8392
17824 start http://www.sina.com at1524498288.8397
17825 start http://www.sina.com at1524498288.8407
17820 stop http://www.sina.com at:1524498289.5795 cost:0.7415988445282
17824 stop http://www.sina.com at:1524498289.5899 cost:0.7502818107605
17822 stop http://www.sina.com at:1524498289.5968 cost:0.75795722007751
17821 stop http://www.sina.com at:1524498289.6514 cost:0.81310606002808
17823 stop http://www.sina.com at:1524498289.6571 cost:0.81785297393799
17825 stop http://www.sina.com at:1524498289.6614 cost:0.820631980896
All request handle stop at 1524498289.6657 cost:0.83276891708374

单进程测试：
All request handle stop at 1524498293.6477 cost:3.9819581508636

可以看到，多进程比单进程耗时要小的多。我的服务器是阿里云上的 1G 内存，单核 cpu,不过是 64 位的，不知道这个是否有影响。麻烦 V 友解答，谢谢。运行代码，需要给 php 加上 pcntl 扩展。